Showing papers in "European Physical Journal A in 1992"

Parton distributions for high energy collisions

Abstract: Recent data from deep inelastic scattering experiments atx≳10−2 are used to fix the parton distributions down tox⋟10−4 andQ2⋟0.3 GeV2. The predicted extrapolations are uniquely determined by the requirement of avalence-like structure ofall parton distributions at some low resolution scale and are furthermore shown to be insensitive in the small-x region, 10−4≲x≦10−2, to the detailed experimental input at the presently accessiblex>10−2. Simple parameterizations of the resulting parton distributions are presented in the range 10−5≲x<1 and 0.3≲Q2≲108 GeV2 as obtained from the leading- and higher-order evolution equations.

How well do we understand the synthesis of heavy elements by heavy-ion induced fusion?

Abstract: Experimental cross sections to synthesize heavy ion induced fusion reactions are confronted with calculations using the conventional picture of complete fusion after passing the barrier plus statistical deexcitation after complete equilibration. The sensitivity of the calculations to some of the parameters is demonstrated. Despite this sensitivity, a single parameter set is found that reproduces well the large body of data from experiments using actinide targets. By comparison, for the more symmetric entrance channels using targets around208Pb, we find a fusion hindrance that increases steeply with increasing compound nuclear charge. Predictions for a few reactions of future interest for the synthesis of heavy elements are also presented.

Finding jets and summing soft gluons: A New algorithm

Abstract: We examine a new algorithm for finding jets ine+e− annihilation, using a jet measure based on relative transverse momentum. We perform an analytic calculation of the three-jet fraction at lowest order, and compare our result with the standard jet-finding algorithm. For soft gluons in an abelian theory it is shown that the leading double logarithms exponentiate, unlike the situation for the commonly used algorithm based on invariant mass. In QCD we find that there are leading non-abelian logarithms, and we calculate these explicitly atO(αs2). We discuss the modifications to the algorithm which are needed when the mass of a parton cluster is taken into account. The hope is that the new algorithm will allow an improved theoretical analysis at smaller values of the resolution parameteryT, and hence an improved fit to the experimental data.

Isotopic dependence of electron screening in fusion reactions

Abstract: The fusion reactions6Li(p, α)3He,6Li(d, α)4He, and7Li(p, α)4He have been studied over the c.m. energy rangeE=10 to 1450 keV. Each reaction involved the use of hydrogen projectiles and LiF solid targets as well as Li projectiles and hydrogen molecular gas targets. In all cases the effects of electron screening on the low-energy fusion cross sections (exponential enhancement) have been observed; the effects are somewhat stronger in the case of atomicp ord projectiles compared to the case of molecularH 2 orD 2 gas targets. If isotopic effects on electron screening are negligible, all three reactions should exhibit the same enhancements for each set of experimental techniques. The measurements confirmed this expectation to a large extent.

Electroweak radiative corrections to e + e - → HZ

Abstract: The complete electroweak O(α) radiative corrections to the Higgs-boson production processes e + e − → νllH (l = e, �, τ ) are calculated in the electroweak Standard Model. For e + e − → νe¯ νeH, where ZH production and W-boson fusion contribute, both production channels are added coherently. The calculation of the corrections is described in some de- tail including, in particular, the treatment of the Z-boson resonance in the ZH-production channel. The discussion of numerical results focusses on the total cross section as well as on angular and energy distributions of the Higgs boson. In the G� -scheme, the bulk of the corrections is due to initial-state radiation. The corrections turn out to reduce the total cross section by ∼ 10% for high energies, where the W-boson fusion dominates. In this region, the corrections depend only weakly on the energy and the production angle of the Higgs boson. Based on an analysis of the leading universal corrections, a simple improved Born approximation is introduced. This approximation describes the corrected cross section within about 3%.

Proton-neutron quasiparticle RPA with separable Gamow-Teller forces

Abstract: A comprehensive representation is presented of a generalized form of the proton-neutron quasiparticle RPA model, originally introduced by Halbleib and Sorensen almost thirty years ago. The model uses separable Gamow-Teller forces, including, in addition to the particle-hole force of the former model, the particle-particle force, which is of decisive importance for β+ decay andββ decay. The above model has further been extended to the treatment of odd-odd nuclei. An extension is also made to transitions from nuclear excited states. This is essential for calculations of nuclear weak transition rates in the high-temperature interior of massive stars. Complementing the discussion of Halbleib and Sorensen on the particle-hole force, the structure of the RPA dispersion relation is discussed with emphasis on effects of the particle-particle force.

Ground-state properties of neutron-deficient platinum isotopes

Abstract: The hyperfine structure splitting and the isotope shift in the λ=266 nm transition of Pt isotopes within the mass range 183 ≦A≦ 198 have been determined by Resonance Ionization Mass Spectroscopy (RIMS) in combination with Pulsed-Laser Induced Desorption (PLID). The Pt isotopes were obtained at the on-line isotope separator ISOLDE-3/CERN as daugthers of the primarily produced Hg isotopes. Magnetic moments, quadrupole moments, and changes in the mean-square charge radii are deduced and compared with results of a particle-triaxial rotor model and mean field calculations. Good agreement with experimental data (including nuclear level schemes and transition probabilities) can only be obtained if triaxial shape is admitted. The calculations yield a smooth transition in the shape of odd-A Pt nuclei from a slightly deformed, nearly oblate195Pt via triaxial197-187Pt to a strongly deformed nearly prolate177Pt.

An Experimental study of the process gamma gamma ---> pi+ pi-

Abstract: Using the CELLO detector, we have measured cross sections for the processγγ→π + π − in the mass range 0.75–1.9 GeV/c2. A partial wave fit to the data indicates the presence of a sizeableS-wave amplitude with signs of resonant behaviour. Values for the γγ width of thef 2(1270) are given, showing the model-dependence of this quantity. At higher dipion masses, the model of Brodsky and Lepage is found to give an order of magnitude description of the data.

Combining a langevin description of heavy-ion induced fission including neutron evaporation with the statistical model

Abstract: A model is developed which combines dynamical (Langevin-) calculations with the Kramers modified statistical model in order to describe heavy-ion induced fission including neutron evaporation. In the example of the19F+181Ta collision, the energy dependence of fission probabilities, neutron multiplicities and (H.I.,xn)-cross sections is calculated and a fair agreement with the data is achieved with a reduced friction parameter ofβ=3*1021sec−1. We pay particular attention to the angular momentum dependence of the fusion-fission process.

The pick-up mechanism in composite particle emission processes

Abstract: The pick-up mechanism has been included in the exciton model for the light composite particle emissions. Based on the cluster phase space integration method the formation probabilities of α,d, t,3He are obtained. The calculation results of (n, t) cross sections indicate that this theoretical method can reproduce the experimental data nicely. For triton emissions in pre-equilibrium reaction processes, the semi-direct reactions are the dominant terms which are just omitted in the previous model calculation.

Measurement of hot spots inside the proton at HERA and LEP/LHC

Abstract: In this paper we discuss the measurement of regions in the proton where the density of small-x partons is large, called “hot spots”, by means of an associated jet analysis. An analytical estimate of the cross section is presented and the jet kinematics is discussed in the HERA and LEP-LHC frame. A Monte Carlo estimate shows that the number of jets produced in deep inelastic scattering events at HERA, suitable for this analysis, amounts to a few 1000 jcts for a data sample with an integrated luminosity of about 10 pb−1.

On the relation between protonium level shifts and nucleon-antinucleon scattering amplitudes

Abstract: We study the Coulomb and threshold corrections to the low-energy scattering parameters for the proton-antiproton system, with numerical examples based on current optical models. Some commonly used expressions are shown to be inconsistent with our results. We check that the Trueman formula is valid for the level shifts of protonium, provided one takes properly into account the Coulomb corrections to the scattering lengths and volumes.

Signals of new gauge bosons at future e+ e- colliders

Abstract: We analyze possible indirect signals of additional neutral gauge bosons at futuree + e − colliders, concentrating onSU(2) L ×U(1) y ×U(1) y , andSU(2) L ×SU(2) R ×U(1) effective theories We develop a simple formalism to describe these effects and make a careful study of radiative corrections, in particular initial state radiation, which will be shown to have important implications To make realistic estimates of the sensitivity to the new gauge boson effects, we use a model detector fore + e − annihilation at a center of mass energy of 500 GeV Using a number of selected physical observables we then show that masses considerably higher than the total energy (up to a factor of 6) can be probed and that distinction between various theoretical models is possible

The energy levels of the heavy flavour baryons in the topological soliton model

Abstract: The energy levels of the charm and bottom as well as the mixed flavour hyperons are calculated with the model in which the hyperons are described as bound states of a topologicalSU (2) soliton andK-, D- andB-mesons. The spectra are obtained in a modified version of the Skyrme model where the chiral symmetry breaking term in the Lagrangian density is modified so as to incorporate the different values of the decay constants of the mesons of different flavour. The predicted strange and charmed hyperon spectra are in very good agreement with the empirical values, while the bottom hyperon energies that are more sensitive to the short range dynamics are somewhat below the empirical values. The predicted hyperfine spectra are remarkably close to those obtained with the constituent quark model, more or less independently of the short-distance properties of the effective Lagrangian. We suggest that this feature reflects the presence of an induced nonabelian gauge potential generated by the interplay between “fast” and “slow” degrees of freedom in the meson-soliton system.

Precision measurement of the conversion electron spectrum of 83mKr with a solenoid retarding spectrometer

Abstract: This paper reports on precision measurements of conversion lines in the decay of83mKr with nuclear transition energies of 32.1 keV and 9.4 keV, respectively. The spectra were taken from a submonolayer surface of83m Kr frozen onto a cold backing, using the new Mainz solenoid retarding spectrometer. The high luminosity and resolution of this instrument enables the observation of all allowed conversion lines up to theN-shell and to fully separate the elastic component from inelastic satellites. The combined analysis of the data yields the transition energiesE y=32151.5±1.1 eV and 9405.9±0.8 eV, respectively. The experiment served also to pilot the application of this spectrometer to the question of a finite neutrino rest mass, searched for in theβ-decay spectrum of tritium and to problems in precision electron spectroscopy in general.

New neutron-rich isotopes in the scandium-to-nickel region, produced by fragmentation of a 500 MeV/u 86 Kr beam

Abstract: We have measured production cross-sections of the new neutron-rich isotopes58Ti,61V,63Cr,66Mn,69Fe,71Co and neighbouring isotopes that have been identified as projectile fragments from reactions between a 500 MeV/u86Kr beam and a beryllium target. The isotope identification was performed with the zero-degree magnetic spectrometer FRS at GSI, using in addition time-of-flight and energy-loss measurements. The experimental production cross-sections for the new nuclides and neighbouring isotopes are compared with an empirical parametrization. The resulting prospects for reaching even more neutron-rich isotopes, such as the doubly-magic nuclide78Ni, are discussed.

The decay modes of proton drip-line nuclei with A between 42 and 47

Abstract: Neutron-deficient isotopes with Z=21 to 26 have been produced as projectile-like fragments of an intense 58Ni GANIL beam of 69 MeV/nucleon. The nuclei selected by the upgraded LISE3 spectrometer were identified and implanted in a silicon detector telescope. The 43Cr, 47Fe and 46Fe isotopes were identified for the first time whereas 45Fe, 45Mn, 44Mn and 42V were not observed, indicating probable instability of these nuclei against particle emission. Measurements of the half-lives of 43Cr and 46Mn have been performed and the analysis of their measured beta-delayed proton spectra has given, through the isobaric multiplet mass equation, an empirical estimation of their masses. Half-lives of 44Cr, 43V, 47Fe and 46Fe have also been measured. A discussion of various mass predictions for nuclei at the proton drip-line is given.

Charge-changing cross sections of the neutron-rich isotopes8,9,11Li

Abstract: Total charge-changing cross sections have been measured for8Li on C and Pb targets, for9Li on C, Al, Cu, Sn and Pb targets, as well as for11Li on C, Sn and Pb targets at about 80 MeV/nucleon. These data are compared to measured total reaction cross sections and Glauber-type calculations using Hartree-Fock density distributions. These comparisons allow to draw conclusions on the proton density distribution of the neutronrich lithium isotopes. The results show that even for the most exotic nucleus11Li the proton distribution is only very weakly influenced by the long tail in the neutron density distribution already established in several experiments.

Potential energy and fission barriers of superheavy nuclei calculated in multidimensional deformation space

Abstract: The ground-state collective potential energy of even-even superheavy nuclei with the proton numberZ=112–130 and the neutron numberN=152–210 is analyzed in a multidimensional deformation space. The energy is calculated by the macroscopic-microscopic method. The Yukawa-plus-exponential model is used for the macroscopic part of the energy and the Strutinski shell correction, based on the Woods-Saxon single-particle potential, is taken as the microscopic part. Three axial-symmetric deformations:β 2,β 4,β 6, and quadrupole and hexadecapole non-axial deformations are taken into account. The spontaneous-fission and alpha-decay half-lives are also calculated. Rather low fission barriers (3–7 MeV lower than calculated previously) are obtained, especially when non-axial shapes of a nucleus are considered. Nevertheless, the calculated fission lifetimes are larger than the alpha-decay half-lives for most of considered nuclides. A large number of nuclides withZ=112–122 is predicted to have the total half-life long enough to be detected, when synthesized in the laboratory.

Doppler spectroscopy on relativistic particle beams in the light of a test theory of special relativity

Abstract: Modern experimental techniques based on collinear laser spectroscopy have led to greatly improved versions of the classical Ives-Stilwell experiment. The new experiments employ collinear laser beams and measure the Doppler shift as it is seen in the rest frame of a relativistic particle beam. This note analyzes these experiments in the framework of a test theory of special relativity due to Mansouri and Sexl. It is found that the experiments can test the coefficient functiona(υ2 (responsible for time dilation), but are insensitive tob,d andɛ. While the Mansouri-Sexl theory in principle allows for an anisotropy of the velocity of light with respect to a moving reference frame, anisotropy effects cancel out in Ives-Stilwell type experiments. These experiments by themselves are not sufficient for a direct observation of such an anisotropy.

Mass spectroscopy of13Be

Abstract: The mass of13Be has been measured with the reaction13C(14C,14O)13Be at ELab=337 MeV. A Q-value of Q0=−37.02(5) MeV was obtained and the mass excess is M.E.=35.16(5) MeV. If the observed line corresponds to the ground state,13 Be is particle unstable with respect to the oneneutron emission by 2.01 MeV. The observed line width of 0.3(2) MeV supports an assignment ofJπ=5/2+ or 1/2−, but excludesJπ=1/2+. An excited state is seen at 3.12(7) MeV; there are indications of a second excited state at 6.5(2) MeV.

Production of D(s)+ mesons in B decays and determination of f(D(s))

Abstract: The production ofDS+ mesons inB meson decays, and inq\(\bar q\) continuum events, has been studied with the ARGUS detector at thee+e− storage ring DORIS II. In addition to the measurement of inclusiveDS+ production in γ(4S)→B\(\bar B\) decays, all eight two-body decay modesB→DS(*)D(*) have been measured with branching ratios between 1% and 3%. By comparing our inclusive and exclusive results to predictions of heavy quark effective theory, a value of (267±28) MeV × [2.7%/BR(Ds+→φπ+)]1/2 is obtained for the weak decay constant fDS(*), averaged overDS+ andDS*+ mesons.

Cold multi-nucleon transfer between heavy nuclei and the synthesis of new elements

Abstract: Experimental results of multi-nucleon transfer reactions on very heavy targets (208Pb,235U,248Cm) are analysed with respect to their properties as cold quasielastic reactions. Optimum conditions for quasi-elastic transfer are discussed for the production of very heavy elements.

Comparison of higher order deformations in several s-d shell nuclei obtained through Schrödinger or Dirac coupled-channel analysis of 800 MeV polarized protons inelastic scattering experiments

Abstract: Cross-sections and analyzing powers data obtained by inelastic scattering of 800 MeV polarized protons from severals-d shell nuclei have been analyzed using the Dirac coupled-channel (CC) formalism putting emphasis in the precise determination of higher order deformation in these nuclei. The superiority of the Dirac formalism compared to the classical Schrodinger CC calculations in the analysis of these data was once more clearly established. The higher order deformations obtained through these calculations are compared to those obtained using the classical Schrodinger equation.

High-spin States in the Transitional Nucleus 88Mo

Abstract: The reaction58Ni(36Ar,α qρ)88Mo has been studied at 145 MeV beam energy. A detector array consisting of the OSIRIS spectrometer, four charged-particleΔE detectors and seven NE213 neutron detectors has been used to meaure the gamma radiation inγγ- and particle-γγ-coincidence mode. The level scheme of88Mo has been extended up to 11.6 MeV excitation energy and probable spin 23ħ; some 70 transitions and 40 levels have been identified. Spin assignments have been proposed on the basis of measured DCO ratios. Hartree Fock cranking calculations of the Total Routhians and shell model calculations of the high spin states are presented which imply near-sphericity of the yrast line up to the highest spins found. A classification of the high spin states according to their leading seniority is proposed.

The baryonic spectrum in a constituent quark model including a three-body force

Abstract: We analyze, within a non-relativistic quark model, the low energy part of the baryonic spectrum in the octet and decuplet flavour representations. The relevance of a strong Coulomb potential is emphasized in order to explain its general features. The addition of a three-body force allows to solve the ‘Roper puzzle’, giving a consistent explanation to its relative position in the spectrum.

Atomic effects on α-α scattering to the 8 Be ground state

Abstract: The α-α scattering to the 8Be ground state was investigated in kinematic coincidence for the angle pairs 45‡/45‡ and 30‡/60‡ using the crossing beams technique, with an energy resolution of 26 eV The nuclear resonance is split into 2 structures, not the 3 structures suggested by earlier work The atomic physics origin of the splitting, the resonance parameters of the 8Be ground state, and astrophysical implications are discussed

Precision measurement of antiprotonic hydrogen and deuterium X-rays

Abstract: X-rays from antiprotonic hydrogen and deuterium have been measured at low pressures Using the cyclotron trap, a 105 MeV/c antiproton beam from LEAR was stopped with an efficiency of 86% in 30 mbar hydrogen gas in a volume of only 100 cm3 The X-rays were measured with Si(Li) detectors and a Xe-CH4 drift chamber The strong interaction shift and broadening of the Lymanα transition and the spin-averaged 2p width in antiprotonic hydrogen was measured with unprecedented accuracy The triplet component of the ground state in antiprotonic hydrogen was determined for the first time

High spin states in66,68Ge

Abstract: High spin states of66,68Ge have been investigated at the FN Tandem accelerator of the University of Koln via the reactions40Ca(32S,α2p,4p)66,68Ge at a beam energy of 100 MeV and58Ni(16O,α2p)68Ge at 65 MeV. The OSIRIS spectrometer with 12 escape suppressed Ge detectors was used to measure γγ coincidences and γ-ray angular distributions. In66Ge (68Ge) 33 (22) new levels were found and 63 (62) new γ-transitions were placed in the level scheme. Both nuclei show a rather complicated but similar excitation pattern, ruled by the interplay of quasiparticle and collective degrees of freedom. The results are compared to the recently published EXVAM calculations for68Ge.